"s it possible to select multiple layers in Gimp?"
No..and not in any graphic editor i know
"Color Overlay effects to more than one layer, is t possible?"
should be possible by editing the script but for that i can't help , sorry,
maybe somebody else here can do

This is one of the problems I have with the Gimp. If I have two or more layers I want to copy at the same time to achieve different results with I am forced to copy EACH individual layer and then move them into the order in which I want them to be in. A time wasting process which stifles my productivity. I like Gimp for a number of things but this extreme lack of functionality along with the glaring lack of Adjustment layers is a serious mistake on the part of the developers. I'm going back to Photoshop for my photo processing needs.

The current beta of GIMP has layer groups, but you are right. GIMP needs to be able to copy multiple layers. Still, I've only had rare occasions when I really wished I had this ability (and of course, did a lot of work to work around this). :)